cmd/go: permit wrongly rejected -Wl,-O... linker flags

A typo caused the validation rule to check against -WL,-O... which is
not a regular flag because the L should be lowercase as in the other
rules. This caused valid linker flags to be rejected and people had to
work around this by filtering their default flags that include, e.g.,
-Wl,-O1 for a simple link optimization.
Fix the typo that wrongly rejected -Wl,-O... but allowed a non-existing
-WL,-O flag.

Change-Id: Ia3bf730f16f5ad98a39d7f17159de17b44075462
GitHub-Last-Rev: 2ec7f2a2b9
GitHub-Pull-Request: golang/go#42631
Reviewed-on: https://go-review.googlesource.com/c/go/+/270278
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/work/security.go

@@ -179,7 +179,7 @@ var validLinkerFlags = []*lazyregexp.Regexp{
 	re(`-Wl,-berok`),
 	re(`-Wl,-Bstatic`),
 	re(`-Wl,-Bsymbolic-functions`),
-	re(`-WL,-O([^@,\-][^,]*)?`),
+	re(`-Wl,-O([^@,\-][^,]*)?`),
 	re(`-Wl,-d[ny]`),
 	re(`-Wl,--disable-new-dtags`),
 	re(`-Wl,-e[=,][a-zA-Z0-9]*`),